1. Field of the Invention
The present invention relates to a composite sheet for impervious coating of construction components which are covered with a water-repellent hydrocarbon material. It relates more particularly to a composite sheet for impervious coating of roofing components covered with a material based on bitumen or expanded polystyrene. It also relates to a process for the manufacture of such composite sheets.
2. Description of the Related Art
At the present time impervious coating of construction components which are continuously exposed to the action of atmospheric agents (rain, snow, sunlight, wind etc) is largely provided by means of sheets of waterproof thermoplastic materials which to a greater or lesser extent stand up well to inclement weather and to the longterm effect of ultraviolet rays. In most cases, these are sheets based on flexible polyvinyl chloride, which have both the strength and the flexibility which are necessary for this utilization.
However, until now, it has not been possible to use sheets based on polyvinyl chloride for coating construction components covered with water-repellent hydrocarbon materials such as materials based on bitumen, tar or asphalt, or materials based on expanded polystyrene.
These sheets based on flexible polyvinyl chloride contain, in fact, a relatively large quantity of a plasticizer, which is, in most cases, a fairly volatile organic ester which is gradually extracted from these sheets by the said hydrocarbon materials. As a result, these sheets become hard and brittle and they crack and can no longer provide the construction components which they cover with any imperviousness at all (Kalle Patent Application DE-A No. 1,184,481, column 1, lines 1 to 18).
Attempts have already been made to overcome this disadvantage in many ways. Thus, it has been proposed to replace at least a part of the polyvinyl chloride in these sheets with chlorinated polyethylene (abovementioned patent application). This attempt, which involves the manufacture and the mixing of polymers of two different types, the of which (polyethylene) needs, moreover, to be mod chlorination, is costly and has not been found wholly satisfactory. Attempts have also been made to replace the monomeric plasticizers of polyvinyl chloride, which are in most cases esters derived from monohydric alcohols, polymeric plasticizers derived from dicarboxylic acids and from polyhydric alcohols such as propyl and butylene glycols (H. A. Sarvetnick, Polyvinyl Chloride, Reinhold Plastics Applications Series, Van Nostrand Reinhold Company, 1969, page 70).
While these polymeric plasticizers are less volatile and are less liable to be extracted than the monomeric plasticizers, they are costly products which are less effective and which consequently lead to less flexible and less resistant products (op. cit., pages 70 and 71).
Techniques which avoid direct contact between the sheet based on flexible polyvinyl chloride and these materials have also been tried in order to solve the problem of the extraction of plasticizer by hydrocarbon materials. Thus, for example, felts of various kinds have been combined by hot lamination with the face of the sheet which is intended to be placed in contact with the hydrocarbon material; the face of the sheet intended to be placed in contact with the hydrocarbon material has also been coated with varnishes based on acrylic, polyester or fluoropolymer resins. These techniques have not been found entirely satisfactory either, because the felt inserted, or the coating varnish, itself gives rise to plasticizer extraction (op. cit., page 72) or does not sufficiently resist this extraction.
Attempts have also been made to overcome the disadvantages resulting from the gradual extraction of the plasticizer from sheets based on flexible polyvinyl chloride by hydrocarbon covering materials by employing methods which rely on special treatments of the surface of these sheets (abovementioned patent application DE-A No. 1,184,481, column 1, lines 19-21).
However, these methods, too, have been set aside because the resulting improvement involves only an extremely surface layer of the sheet and consequently disappears idly when this layer is damaged by chemical or mechanical agents (idem, column 1, lines 22 to 27).